In the manufacture of composite structures, it is conventional to bond a metallic structure to another metal structure or to a composite structure composed of resin impregnated fibrous reinforcement utilizing structural adhesives. In general, to ensure the greatest level of adhesive strength, the metal surface(s) are scrupulously cleaned of dirt, soil, grease, and metal oxidation products immediately prior to bonding. Unfortunately, this procedure cannot be generally used most times, as the cleaning and bonding operations are often separated by long periods of time. During such periods, the metal surface may become hydrolyzed, lessening the adhesive strength of the bond. One alternative to overcome this difficulty is to use a primer on the cleaned metal surface.
Historically, chromated primers (i.e., solutions containing chromium ions) have been used to protect metals from corrosion. However, due to environmental regulations, the use of chromates is restricted, particularly in the aerospace industry, among others. Several non-chromated corrosion inhibitors such as zinc phosphosilicates, molybdenum zinc phosphate, calcium borosilicate, sodium vanadate, strontium phosphate etc. have been under evaluation. Most of these inhibitors are passive (cannot leach-like chromates) and provide corrosion protection by sacrificial oxidation method. As such, these passive inhibitors do not provide the desired durability or performance required when exposed to harsh environmental conditions.
Some conventional organic corrosion inhibitors rely on a mechanism through which organic species prevent corrosion is by reacting with the metal substrate, the oxide film or the corrosion products to form an adherent film to prevent further corrosion. A major drawback of these organic corrosion inhibitors relates to the interaction of the functional groups used to form strong adherent bonds on a metal substrate with the primer formulation. Due to this interaction, the shelf life and cure kinetics of the primer may be affected, which limits corrosion inhibitor transport within a coating to the corrosion site. Another drawback with many organic corrosion inhibitors is their unpredictable corrosion performance when used with epoxy based corrosion inhibiting primer formulations in preventing corrosion on highly corrosive material such as aluminum and aluminum alloys.
Therefore, there remains a need for non-chromated primer formulations that can perform similarly to chromate corrosion inhibitors for structural bonding applications, particularly in industries such as aerospace and automotive.